Category Archives: WDM Optical Network

Passive Optical Network Technology

The tremendous growth in IP traffic badly influenced the access network capacity. It’s believed that the copper-based access networks can’t provide either the minimum bandwidth or the required transmission distance for delivering services of voice, data, and video programs. Passive optical network (PON) is seemed as a promising and cost-effective way to solve this problem.

What’s PON?

PON is a telecommunication network that uses point-to-multipoint fiber to the end-points in which optical splitters are used to enable a single optical fiber to serve multiple end-points. It does not include any electrically powered switching equipment.

Three Devices in PON

There are three distinct devices in the network (as shown in the following picture): the OLT (optical line terminal), the ONUs (optical network units) or ONTs (optical network terminals) and the splitter. Each one has a necessary function in the passive optical network. PON always works under transmission between the OLT and the different ONT’s through optical splitters, which multiplex or demultiplex signals based on their origin and destination.


  • OLTs are located in provider’s central switching office. This equipment serves as the point of origination for FTTP (Fiber-to-the-Premises) transmissions coming into and out of the national provider’s network. An OLT, is where the PON cards reside.
  • ONU converts optical signals transmitted via fiber to electrical signals. These electrical signals are then sent to individual subscribers. ONUs are commonly used in fiber-to-the-home (FTTH) or fiber-to-the-curb (FTTC) applications. Using different wavelengths for each service makes it possible to transmit high-speed Internet and video services at the same time. Wavelength multiplexing is performed at the central office and a wavelength demultiplexing mechanism is provided at the customer’s house.
  • PON splitter is used to split the fiber optic light into several parts at a certain ratio. For example, a 1X2 50:50 fiber optic splitter will split a fiber optic light beam into two parts, each get 50 percent of the original beam.
Advantages of PON

There are many advantages given by the use of fiber and the passive elements that compose the network. The following will tell about the advantages of PON.

  • High bandwidth The bandwidth allowed by systems based on PON can reach the 10 Gbps rate down to the user. The need to increase the bandwidth and the speed is another justification for the use of PON.
  • Long distance A PON allows for longer distances between central offices and customer premises. While with the Digital Subscriber Line (DSL) the maximum distance between the central office and the customer is only 18000 feet (approximately 5.5 km), a PON local loop can operate at distances of over 20 km.
  • Low cost On one hand, the cost of passive elements is low. On the other hand, the installation of these PON elements is much more economic. And it avoids operation and maintenance costs, such as absence of falls or maintenance of the network feeds.

Of course PON has some disadvantages. Compared with an active optical network, it has less range. That means subscribers must be geographically closer to the central source of the data. PON also make it difficult to isolate a failure when they occur. However, these disadvantages can not avoid choosing PON as the best possible configuration. Because it saves the cost of deploying PON networks regarding other two configurations (point to point and active optical network). And the flexibility of the network allows the usage of a channel by a large number of users.

Knowledge About ROADM

Over the past decades, the use of bandwidth in transport networks increases dramatically. This resulted in the generation of wave division multiplexing (WDM) which could introduce more bandwidths on a single fiber. The need for more bandwidth flexibilities, operational efficiencies, and technology advances brought the optical add/drop multiplexer (OADM) to add or drop wavelengths at a node point. FOADM, an initial type of OADM, which uses fixed lasers for fixed wavelengths has emerged. But it can’t meet the needs of bandwidth. So this drives the emergence of the other kind of OADM – ROADM.

ROADM (reconfigurable optical add/drop multiplexer) adds the ability to remotely switch traffic from a WDM system at the wavelength layer. It avoids the unnecessary optical-electrical-optical conversion. And it’s bit-rate/protocol transparent, so future upgrades of bit-rate/protocol can be accommodated without upgrading the switch. ROADM technology has revolutionized optical network and offered huge bandwidth for data transport.

ROADM is an integral part of WDM networks due to its advantages. ROADM allows for remote configuration and reconfiguration. The planning of entire bandwidth assignment does not need to be carried out during initial deployment of a system. The configuration can be done as required without affecting traffic already passing the ROADM. It provides full flexibility of delivering any wavelength to any node throughout the ring infrastructure. It automates the optical layer to remove error-prone service provisioning, and equalizes signal loss across all wavelengths, reducing the need for expensive signal boosting equipment. What’s more, it can reduce the costs of networks.

ROADM functionality firstly appeared in long-haul equipment. By 2005, it started to appear in metro optical systems because of the network traffic driven by the increasing demand for packet-based services such as Ethernet, high-speed data, audio and video services. ROADM equipment is used to build a versatile, agile and quickly provisioned optical transport network. This transport network can scale in both distance and number of nodes.

As the continuous development of technology, it brings three generations of ROADM. The 1st generation was to solve fiber exhaust problems caused by inflexibility in the long haul networks. Compared with the 1st generation, the 2nd generation ROADM is typified by wavelength blocker technology. And the structure design of the 2nd generation is simple with a tap (a splitter and filter array) used to drop any number of selected wavelengths. But if all wavelengths enter the blocker, the pass-through wavelengths will not be blocked. The 3rd generation is the wavelength selective switch (WSS) (see the following picture). It’s more versatile, smaller, consuming less power and cheaper than 2nd generation ROADM.


Common advantages of ROADM like automatic performance monitoring and equalization make WDM systems more useful. However, the current ROADM technology is not perfect and still needs to be upgraded. The development of the advanced ROADM depends on the growth of supporting optical components maturity, progress of integrated optics technology, improved capability of the equipment and new algorithms. Whatever, the new design should concern those factors such as the future broadband communication network service needs, fewer components, fewer devices in network, effective interoperability and the flawless service evolution with considerable decrease in the operational costs.